The present invention generally relates to an optical beam splitter and, in particular, relates to an achromatic beam splitter the resultant beams of which maintain a preselected energy ratio regardless of instabilities.
Many conventional analytical instruments employ what is commonly known as a double beam technique for analyzing a sample material. In this technique, one beam is passed through a sample material under test and a second beam is passed through a reference material having known characteristics. Both beams are detected by known means and compared. By such a comparison, information about the sample material carried by the beam passed therethrough is easily discerned and evaluated.
Also, it is often true that it is desirable to pass the sample beam through a cell of the least possible volume for a given path length, whereas this is not important for the reference beam.
Quite often in the double beam technique a single light beam from an arc lamp, or other spectral source, is split into a sample beam and a reference beam. The manner by which the single beam is split, to date, has been either significantly dependent upon the wavelengths over which the splitter is to operate, or affected by relative motion of the beams and the optical elements, or both. That is, a splitter which is operational at longer wavelengths, such as the infra-red spectrum, is often not functional at shorter wavelengths, such as the ultraviolet spectrum. This is, as well known in the optical art, because, for most optical elements, the transmission and reflecting properties vary with wavelength.
One solution used to overcome the above-described difficulties is by positioning a moving light chopper in the path of the single beam and deflect the light beam passing therethrough alternately into either the reference beam path or the sample beam path.
One drawback to using a mechanical light chopper is that the reference beam and the sample beam are spaced apart in time. This time spacing complicates the data processing equipment necessary for comparing the two beams. In addition, the use of a light chopper involves rather complex mechanical and electrical apparatus for the proper and accurate operation thereof. Further, significant signal noise is introduced by the inherent mechanical inaccuracies of the chopper mechanism.